The present invention relates to a power transmission for an automotive vehicle, and more particularly to a power transmission for a four-wheel drive vehicle which includes a change-speed gearing drivingly connected to a prime mover of the vehicle to selectively establish a forward drive power train or a reverse drive power train, and an intermediate differential unit arranged to be applied with a driving torque from the prime mover through the change-speed gearing for transmitting the driving torque to a first drive shaft for front-wheel drive and to a second drive shaft for rear-wheel drive.
In conventional power transmissions of this kind, there have been adapted two kinds of intermediate differential units, one of which is the bevel gear type and the other of which is the planetary gear type. Such conventional intermediate differential units each include an input member drivingly connected to an output shaft of the change-speed gearing, and a single pair of output members arranged to be applied with a driving torque from the change-speed gearing through the input member for transmitting the driving torque to the first and second drive shafts at a predetermined gear ratio. It is, however, noted that the distribution ratio of the driving torque to the drive shafts should be changed in accordance with the road condition to ensure stable maneuverability of the vehicle. For example, if the distribution ratio of the driving torque is determined to ensure stable maneuverability of the vehicle on a road of low frictional coefficient, the vehicle will understeer on a road of high frictional coefficient. If the distribution ratio of the driving torque is determined to ensure stable maneuverability of the vehicle on a road of high frictional coefficient, traction of the vehicle will become insufficient on a road of low frictional coefficient. For this reason, an auxiliary transmission has been adapted to the intermediate differential unit to change the distribution ratio of the driving torque in accordance with the road condition. Such arrangement of the auxiliary transmission, however, results in increase of the size and weight of the power transmission assembly.